1. Field of the Invention
The present invention is related to wireless communications networks and, more particularly, to distributing precision timing throughout a wireless network.
2. Background Description
A number of applications currently exist within wireless communication systems, such as those supporting Global System for Mobile Communication (GSM), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) and Universal Mobile Telecommunications System (UMTS) technologies, for which precise common timing information is needed by mobile units and by other entities in the wireless network. Examples of such applications include Global System for Mobile Communication (GSM) positioning and assisted GPS (A-GPS) positioning. Mobile units with A-GPS acquire and measure signals from a number of GPS satellites in order to obtain an accurate estimate of their current geographic position. It is well known that precise knowledge of GPS time can greatly improve positioning measurements for higher sensitivity in otherwise poor signal areas, e.g., indoors where a GPS satellite signal may be blocked. Another application would be accurate time stamping of significant events (e.g. alarms and faults) by network entities such that events emanating from the same cause but registered in different entities could more easily be associated through their common time of occurrence.
In some wireless technologies, e.g., CDMA, the transmission timing of all base stations has to be precisely and explicitly synchronized to a common time source, such as Global Positioning System (GPS) originated clock. Such a precise transmission timing clock provides wireless terminals with unrestricted access to precise common timing information without any special additional support. In other technologies, like GSM and TDMA, each base station maintains its own local timing source, which, though precise within its own frame of reference, does not indicate a particular universal time nor align with the timing maintained by other base stations.
Providing precise common timing information for GSM, TDMA or UMTS base stations may require deploying additional units, for example Location Measurement Units (LMUs) in GSM or UMTS, that measure and associate the transmission timing of one or more base stations with a common timing source. The precise association of the local timing of each base station with the common timing source can be passed to mobile units and base stations for deriving accurate timing, according to the common timing source, from the local transmission timing of a particular base station—e.g. the base station serving a particular mobile unit. GSM LMUs tend to require additional hardware and are expensive additions in any wireless network. Moreover, in order to synchronize the transmission timing of every wireless network base station with a common timing source, it may be necessary to deploy a separate measurement unit for every base station, or every few base stations, thereby further increasing cost and deployment time.
Thus, there is a need for precise common timing information distributed throughout wireless networks without requiring high cost measurement units and with no impact or minimal impact to mobile units.